This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No(s). 91114641 filed in TAIWAN, R.O.C. on Jul. 2, 2002, which is(are) herein incorporated by reference.
1. Field of Invention
The invention relates to a non-volatile static random access memory (SRAM) cell and, in particular, to a non-volatile SRAM cell that makes use of mask read only memory (MROM).
2. Related Art
There are two main type of memory: the volatile memory and the non-volatile memory. In the development history of non-volatile memory, the first proposed mask read only memory (MROM) first load programs or data into the photo mask used during the manufacturing process. Thus, they can permanently store data. The structure of its cell does not need to be manufactured using any special process. Moreover, there is no writing involved. Therefore, it is very economical and simple.
Nevertheless, there are the following problems to be solved. For instance, the time it takes for the memory to be made in the wafer fab and to be delivered to the customer is longer than other types of ROM (such as EPROM or EEPROM). This is mainly because the MROM is a configure-to-order (CTO) product. The fab starts preparing production only after receiving the memory codes sent from the customer.
On the other hand, the photo mask has to be manufactured according to the ROM asked by the customer. It is not finished until a certain chip process is done. Therefore, its cost for small-quantity production tends to be higher. Once the manufacturing is over, there is no way to modify its internal memory data. As a result, there are some risks for both the customer and the manufacturer.
Although there are the above-mentioned problems, the MROM still has its practical values. Driven by the needs for consuming electronics, memory of different specifications and types exist on the market. One of them is the non-volatile SRAM. The proposal of this type of technology is because the SRAM is volatile memory. Once if the power is turned off, the data stored there in disappear immediately. To compensate for this drawback, the non-volatile SRAM with memory properties becomes the modern trend.
Since electronic products such as PDA""s and game machines are getting more compact, the chip area plays an important role. However, the prior art has the SRAM unit and the MROM cell on separate regions of a chip. Therefore, it is very inefficient. Thus, it is of great importance to provide a non-volatile SRAM cell with a smaller area.
An objective of the invention is to provide a new memory cell combination, including a SRAM unit and a MROM unit, for selectively being used as SRAM and ROM.
Another objective of the invention is to provide a SRAM cell with a smaller area. The active area where the source contact (Vss contact) is located extends to have a cross intersection area with the polysilicon area where access transistors are formed, constituting a MROM unit. Such a layout enables the word line to be shared between the Vss contact of the SRAM unit and the MROM. They can also be shared in the X-decoder circuit. One does not need to design the two unit circuits in two separate areas, saving the chip area by about 20%.